Life as a Physicist

Ok. This post is for all my non-physics friends who have been asking me… What just happened? Why is everyone talking about this Higgs thing!?

It does what!?

Actually, two things. It gives fundamental particles mass. Not much help, eh? Fundamental particles are, well, fundamental – the most basic things in nature. We are made out of arms & legs and a few other bits. Arms & legs and everything else are made out of cells. Cells are made out of molecules. Molecules are made out of atoms. Note we’ve not reached anything fundamental yet – we can keep peeling back the layers of the onion and peer inside. Inside the atom are electrons in a cloud around the nucleus. Yes! We’ve got a first fundamental particle: the electron! Everything we’ve done up to now says it stops with the electron. There is nothing inside it. It is a fundamental particle.

We aren’t done with the nucleus yet, however. Pop that open and you’ll find protons and neutrons. Not even those guys are fundamental, however – inside each of them you’ll find quarks – about 3 of them. Two “up” quarks and a “down” quark in the case of the proton and one “up” quark and two “down” quarks in the case of the neutron. Those quarks are fundamental particles.

The Higgs interacts with the electron and the quarks and gives them mass. You could say it “generates” the mass. I’m tempted to say that without the Higgs those fundamental particles wouldn’t have mass. So, there you have it. This is one of its roles. Without this Higgs, we would not understand at all how electrons and quarks have mass, and we wouldn’t understand how to correctly calculate the mass of an atom!

Now, any physicist who has made it this far is cringing with my last statement – as a quick reading of it implies that all the mass of an atom comes from the Higgs. It turns out that we know of several different ways that mass can be “generated” – and the Higgs is just one of them. It also happens to be the only one that, up until July 4th, we didn’t have any direct proof for. An atom, a proton, etc., has contributions from more than just the Higgs – indeed, most of a proton’s mass (and hence, an atom’s mass) comes from another mechanism. But this is a technical aside. And by reading this you know more than many reporters who are talking about the story!

The Higgs plays a second role. This is a little harder to explain, and I don’t see it discussed much in the press. And, to us physicists, this feels like the really important thing. “Electro-Weak Symmetry Breaking”. Oh yeah! It comes down to this: we want to tell a coherent, unified, story from the time of the big-bang to now. The thing about the big-bang is that was *really* hot. So hot, in fact, that the rules of physics that we see directly around us don’t seem to apply. Everything was symmetric back then – it all looked the same. We have quarks and electrons now, which gives us matter – but then it was so hot that they didn’t really exist – rather, we think, some single type of particle existed. Now, and the universe cooled down from the big bang, making its way towards present day, new particles froze out – perhaps the quarks froze out first, and then the electrons, etc. Let me see how far I can push this analogy… when water freezes, it does so into ice crystals. Say that an electron was one particular shape of ice crystal and a quark was a different shape. So you go from a liquid state where everything looks the same – heck – it is just water, to a solid state where the ice crystals have some set of shapes – and by their shape they become electrons or quarks.

Ok, big deal. It seems like the present day “froze” out of the Big Bang. Well, think about it. If our current particles evolved out of some previous state, then we had sure as hell be able to describe that freezing process. Even better – we had better be able to describe that original liquid – the Big Bang. In fact, you could argue, and we definitely do, that the rules that governed physics at the big bang would have to evolve to describe the rules that describe our present day particles. They should be connected. Unified!! Ha! See how I slipped that word in up above!?

We know about four forces in the universe: the strong (holds a proton together), weak (radioactive decay is an example), electro-magnetism (cell phones, etc. are examples), and gravity. The Higgs is a key player in the unification of the weak force and the electro-magnetic force. Finding it means we actually have a bead on how nature unifies those two forces. That is HUGE! This is a big step along the way to putting all the forces back together. We still have a lot of work to do!

Another technical aside. We think of the first role – giving fundamental particles mass – a consequence of the second – they are not independent roles. The Higgs is key to the unification and in order to be that key, it must also be the source of the fundamental particle’s mass.

How long have you been searching for it?

A loooooong time. We are like archeologists. Nature is what nature is. Our job is to figure out how nature works. We have a mathematical model (called the Standard Model). We change it every time we find an experimental result that doesn’t agree with the calculation. The last time that happened was when we stumble upon the unexpected fact that neutrino’s have mass. The time before that was the addition of the Higgs, and that modification was first proposed in 1964 (it took a few years to become generally accepted). So, I suppose you could say in some sense we’ve been looking for it since 1964!

It isn’t until recently, however (say in the late 90’s) that the machines we use have become powerful enough that we could honestly say we were “in the hunt for the Higgs.” The LHC, actually, had finding the Higgs as one of its major physics goals. There was no guarantee – no reason nature had to work like that – so when we built it we were all a little nervous and excited… ok. a lot nervous and excited.

So, why did it take so long!? The main reason is we hardly ever make it in our accelerators! It is very very massive!! So it is very hard to make. Even at the LHC we make one every 3 hours… The LHC works by colliding protons together at a very high speed (almost the speed of light). We do that more than 1,000,000 times a second… and we make a Higgs only once every 3 hours. The very definition of “needle in a haystack!”

Who made this discovery?

Two very large teams of physicists, and a whole bunch of people running the LHC accelerator at CERN. The two teams are the two experiments: ATLAS and CMS. I and my colleagues at UW are on ATLAS. If you hear someone say “I discovered the Higgs” they are using the royal-I. This is big science. Heck – the detector is half a (American) football field long, and about 8 or 9 stories tall and wide. This is the sort of work that is done by lots of people and countries working together. ATLAS currently has people from 38 countries – the USA being one of them.

What does a Cocktail Party have to do with it?

The cocktail party analogy is the answer to why some fundamental particles are more massive than particles (sadly, not why I have to keep letting my belt out year-after-year).

This is a cartoon of a cocktail party. Someone very famous has just entered the room. Note how everyone has clumped around them! If they are trying to get to the other side of the room, they are just not going to get there very fast!!

Now, lets say I enter the room. I don’t know that many people, so while some friends will come up and talk to me, it will be nothing like that famous person. So I will be able to get across the room very quickly.

The fact that I can move quickly because I interact with few people means I have little mass. The famous person has lots of interactions and can’t move quickly – and in this analogy they have lots of mass.

Ok. Bringing it back to the Higgs. The party and the people – that is the Higgs field. How much a particle interacts with the Higgs field determines its mass. The more it interacts, the more mass is “generated.”

And that is the analogy. You’ve been reading a long time. Isn’t this making you thirsty? Go get a drink!

Really, is this that big a deal?

Yes. This is a huge piece of the puzzle. This work is definitely worth a Nobel prize – look for them to award one to the people that first proposed it in 1960 (there are 6 of them, one has passed away – no idea how the committee will sort out the max of 3 they can give it to). We have confirmed a major piece of how nature works. In fact, this was the one particle that the Standard Model predicted that we hadn’t found. We’d gotten all the rest! We now have a complete picture of the Standard Model is it is time to start work on extending the Standard Model. For example, dark matter and dark energy are not yet in the Standard Model. We have no figured out how to fully unify everything we know about.

No. The economy won’t see an up-tick or a down-tick because of this. This is pure research – we do it to understand how nature and the universe around us works. There are sometimes, by-luck, spin-offs. And there are people that work with us who take it on as one of their tasks to find spin offs. But that isn’t the reason we do this.

What is next?

Ok. You had to ask that. So… First, we are sure we have found a new boson, but the real world – and data, is a bit messy. We have looked for it, and expect it to appear in several different places. It appeared in most of them – one place it seems to be playing hide and seek (where the Higgs decays to tau’s – a tau is very much like a heavy electron). Now, only one of the two experiments has presented results in the tau’s (CMS), so we have to wait for my experiment, ATLAS, to present its results before we get worried.

Second, and this is what we’d be doing no matter what happened to the tau’s, is… HEY! We have a shiny new particle! We are going to spend some years looking at it from every single angle possible, taking it out for a test drive, you know – kicking the tires. There is actually a scientific point to doing that – there are other possible theories out there that predict the existence of a Higgs that looks exactly like the Standard Model Higgs except for some subtle differences. So we will be looking at this new Higgs every-which way to see if we can see any of those subtle differences.

ATLAS and CMS also do a huge amount of other types of physics – none of which we are talking about right now – and we will continue working on those as well.

What will you all discover next?

We are coming up on one of those “lucky to be alive to see this” moments. Sometime in the next year we will all know, one way or the other, if the Standard ModelHiggs exists. Or it does not exist. How we think fundamental physics will change. I can’t understate the importance of this. And the first strike along this path will occur on December 13th.

If it does not exist that will force us to tear down and rebuild – in some totally unknown way – our model of physics. Our model that we’ve had for 40+ years now. Imagine that – 40 years and now that it finally meets data… poof! Gone. Or, we will find the Higgs, and we’ll have a mass. Knowing the mass will be in itself interesting, and finding the Higgs won’t change the fact that we still need something more than the Standard Model to complete our description of the universe. But now every single beyond-the-standard model theory will have to incorporate not only electrons, muons, quarks, W’s, Z’s, photons, gluons – at their measured masses, but a Higgs too with the appropriate masses we measure!

Ok, this takes a second to explain. First, when we look for the Higgs we do it as a function of its mass – the theory does not predict exactly how massive it will be. Second, the y-axis is the rate at which the Higgs is produced. When we look for it at a certain mass we make a statement “if the Higgs exists at mass 200 GeV/c2, then it must be happening at a rate less than 0.6 or we would have seen it.” I read the 0.6 off the plot by looking at the placement of the solid black line with the square points – the observed upper limit. The rate, the y-axis, is in funny units. Basically, the red line is the rate you’d expect if it was a standard model Higgs. The solid black line with the square points on it is the combined LHC exclusion line. Combined means ATLAS + CMS results. So, anywhere the solid black line dips below the red horizontal line means that we are fairly confident that the Standard Model Higgs doesn’t exist (BTW – even fairly confident has a very specific meaning here: we are 95% confident). The hatched areas are the areas where the Higgs has already been ruled out. Note the hatched areas at low mass (100 GeV or so) – those are from other experiments like LEP.

Now that is done. A fair question is where would we expect to find the Higgs. As it turns out, a Standard Model Higgs will mostly likely occur at low masses – exactly that region between 114 GeV/c2 and 140 GeV/c2. There isn’t a lot of room left for the Higgs to hide there!! These plots are with 2 fb-1 of data. Both experiments now have about 5 fb-1 of data recorded. And everyone wants to know exactly what they see. Heck, while in each experiment we basically know what we see, we desperately want to know what the other experiment sees. The first unveiling will occur at a joint seminar at 2pm on December 13th. I really hope it will be streamed on the web, as I’ll be up in Whistler for my winder ski vacation!

So what should you look for during that seminar (or in the talks that will be uploaded when the seminar is given)? The above plot will be a quick summary of what the status of the experiments. Each experiment will have an individual one. The key thing to look for is where the dashed line and the solid line deviate significantly. The solid line I’ve already explained – that says that for the HIggs of a particular mass if it is there, it must be at a rate less than what is shown. Now, the dashed line is what we expect – given everything was right – and the Higgs didn’t exist at that mass – that is how good we expect to be. So, for example, right around the 280 GeV/C2 level we expect to be able to see a rate of about 0.6, and that is almost exactly what we measure. Now look down around 120-130 GeV/c2. There you’ll notice that the observed line is well above the solid line. How much – well, it is just along the edge of the yellow band – which means 2 sigma. 2 sigma isn’t very much – so this plot has nothing to get very interested yet. But if one of the plots shown over the next year has a more significant excursion, and you see it in both experiments… then you have my permission to get a little excited. The real test will be if we can get to a 5 sigma excursion.

This seminar is the first step in this final chapter of the old realm of particle physics. We are about to start a new chapter. I, for one, can’t wait!

N.B. I’m totally glossing over the fact that if we do find something in the next year that looks like a Higgs, it will take us sometime to make sure it is a Standard Model Higgs, rather than some other type of Higgs! 2nd order effect, as they say. Also, in that last long paragraph, the sigma’s I’m talking about on the plot and the 5 sigma discovery aren’t the same – so I glossed over some real details there too (and this latter one is a detail I sometimes forget, much to my embarrassment at a meeting the other day!).

What you are looking at there is an ACNET plot. I stare at plots similar to this when I’m on shift all the time. The top two plots – the green and red, are position monitors on the quadruple magnets just outside CDF and D0. They are quite stable until the earthquake. The Tevatron was running when this happened, and you can see in that lower red plot that some protons were knocked out of the ring by the ground shaking.

Note these movements are so small you never would have been able to detect them unaided. However, as my wife put it, “that is one expensive seismograph!” 🙂

By now I think most people know how the Prius and other hybrid cards operate. Most cars’ breaks are just like a bycycle break: a clamp that generates a large amount of friction and slows the car down. This is a terrible waste of energy: the car’s motion is converted into heat and damage (to the brake pads) and can never be reclaimed. Think of it as wasted gas, excess pollution, etc.

Hybrids are much more clever. They attach an electric motor/generator directly to the wheel and when you want to break then use the wheel’s motion to run the generator. This requires work – which slows down the car. Instead of the energy being lost, however, it is poured into a battery. The energy can then be reused to get the car started again. Huge savings in gas! This is also why hybrids tend to amazing at city driving, but not long distance driving (where this doesn’t help much because you aren’t stop/start).

Before we got sophisticated with generators and batteries we did something much more mechanical. At least for public transportation: the gyrobus:

Instead of a battery, however, a giant flywheel was used to store the energy. These things were built back in the 1950’s.

Guess what… the same technology has been used for particle accelerators – specifically the Bevatron!

Blah

These are 65 ton flywheels, and there are two of them. Here is an abstract from a paper that describes the control system that ran these puppies:

The Bevatron/Bevalac main guide field power supply stores 680 MJ in the flywheel-shaft systems of two independent motor-generator sets. During the normal acceleration cycle of various heavy-ion beams, the energies of the rotating shafts are converted to energy stored in the main magnet guide field. At the end of the acceleration cycle, the magnet energy is inverted back to the shafts. Generally, this takes place from 10 to 15 times per minute. The rapid switching of ions, energy, and beam lines at the Bevalac has required various control techniques for fast switching between all operational Bevalac fields within 1 min. The power supply control systems and operating parameters are described.

The principle is same as with the hybrid car, or the gyrobus, but all the sizes and power are extreme (as usual for the field of particle physics). Imagine spinning up and down those flywheels at a rate of once every 10 seconds or so! Of course, that system would never have fit in a car!

While I don’t know the answer to this, I suspect that flywheels are still one of the best ways to store energy that has to be quickly extracted over the timescale of seconds. Batteries probably can’t do it without costing a huge amount, and capacitors probably have a much lower energy density – though they are ideal for other stored energy applications that require much faster discharge times!

First, a bit of background on this paper. This is authored by two theorists who analyzed publically released FermiLAT/GLAST data. Fermi is a NASA funded project and one of its stipulations is that all data it collects must be made publically available 6 months after it has been collected. The authors of the paper downloaded the data, used a simple background model, added in their dark matter theory, and did a fit. And pow:

The red points are the data from Fermi, the dash-dot line and the dotted line are backgrounds (galactic diffuse, and a single TeV source), and the dashed line is their model. Nice fit, eh? Yep – looking at this my first reaction is “Wow – is this right? This is big – how did Fermi miss this?” and then I run across the hall to find someone that actually knows this data well.

It turns out the basic problem with this analysis is that not all sources of background are included. This is the galactic center, and, as one would imagine, there are lots of sources there. Not just one TeV source modeled above. My impression from hallway conversations is that when you take into account all of these sources there is much less (if any) room left for the dark matter model. I don’t think that Fermi has published a paper on this yet, but I suspect they will try a some point soon.

Ok, so all’s well. Fermi will publish the paper and everyone will know the right way to do this non-trivial analysis. Except that things got away from them. Nature news has picked it up and wrote a short update. This is pretty widely read. Now Fermi has a PR problem on its hands – people are running around talking about their data and they’ve not really had a voice yet (the science coordinator for Fermi was interviewed for this bit, but her comments were relegated to the end of the post). Fermi is a big collaboration (yes, not the size of the LHC), even if their paper is close to publication it would probably be at least a month or more before the collaboration could agree on a response. So what to do?

There are a lot of issues surrounding making data public. To first order, it is the tax payers that are paying for these experiments, so the data should be public. On the other hand, you can already see that besides the work and infrastructure of making the data public (which costs real $$ – especially for a big experiment like Fermi or one of the LHC experiments), you have to respond to other folks that analyze your data – basically pointing out their mistakes and trying to help them along, even when they might be in competition with some of your internal analyses. In NASA’s case all the data has to be made public – it is written into every grant submission and NASA even provides money for it. This is not currently the case for particle physics. In many of these advanced experiments the data is quite complex – and someone that can’t depend on the large infrastructure of the experiment to help interpret it is bound to have some difficulties.

One only wishes that the authors had gotten in contact with some Fermi folks before submitting their note to the archive…

In addition, Mr. Holder said, the authorities have seized more than $32 million in American currency, 2,700 pounds of methamphetamine, 4,400 pounds of cocaine, 16,000 pounds of marijuana and 29 pounds of heroin. More arrests are expected.

Well… this is what happens when you wait until the evening to write a blog post you spotted in the morning – they change the article. That 2700 pounds? It was 2700 kilograms (which is significantly more). In short – they had mixed kilograms and pounds. I was going to get on my high horse and… well, seems someone at the times is as sensitive about this as us physicists are.

But it also occured to me that the notion of units is rather flexible. For example, when we do particle physics calculations we often set the speed of light to 1. Normally it is 300000000 meters/second (really fast!). Seriously. We just set it to 1. We are so annoyed by having to carry around that number in our calculations that we just up and set it to one. We do that with an other constant as well (called h-bar). Your unit system ends up being very weird when you do that:

Normal Every Day Units

Units in h-bar = c = 1

Energy

Energy

Time

1/Energy

Mass

Energy

Length

1/Energy

I know this seems weird – but you see it all the time. This is just like making the following unit conversion in the list of drugs: instead of telling us the number of pounds or kilograms, tell us how much pot they got in terms of its street value. And to tell the truth, that would have been a very useful number to have in that article.

Heck, in the old days, the unit of measure in the market was the length of the king’s forearm. When the king changed, the whole country would change its unit system…

Un physics professors getting wound up with units is ironic – we don’t really use them that heavily when we get to more advanced calculations. On the other hand, we can only drop them because we have already learned how to use them. At least, that is what we tell ourselves and everyone else! 😉

A few weeks ago the main American particle physics conference, DPF occurred. This is a big conference with lots of plenary and parallel sessions:

At the time I was a short distance away from Detroit, in Ann Arbor, being a Dad. It was a bummer not to be able to attend. I made sure the conference was rendered on my DeepTalk site (picture grab from above). I spent a few lunches the other day browsing it – there are some excellent talks – I definitely recommend checking it out!

This week it is the big Lepton-Photon conference here in Europe. They are simul-casting it as well, so I’m doing my best to watch and record bits of it (more on that in another post). I see someone already submitted that to DeepTalk, so it is partly rendered already. Unfortunately, DeepTalk can’t yet tell that the conference is still ongoing, so it doesn’t automatically update itself. I’ll make sure that happens over the weekend.

It is conference season again. The big one coming up now is Lepton/Photon ‘09 (and yes, I will deeptalk it when it is done!). The run-up to a conference like this is a huge amount of work. The analyzers who are trying to submit analysis are all furiously putting finishing touches on the experiment. And everyone else in the collaboration is busy reviewing these analyses to make sure they don’t have mistakes!

I’m on one of the review boards. It is a lot of work – we regularly get analysis notes that are more than 100 pages in length. As you can imagine, going over them with a fine-tooth comb is a lot of work. And Tevatron analyses are now getting extremely sophisticated. They have to in order to extract every last bit of value from the data rolling out of Fermilab.

I’m used to reading these by now. But I’m always impressed when I listen to other fellow reviewers at how often I miss things. So this time around I’m trying something new to organize my thoughts, comments, etc. A mind map. I’ve used this stuff in the past and found it most useful for organizing a brain-storming session. I’ve never tried to do something this detailed or careful with it before.

It turns out getting a mind map up with questions for the review the first time isn’t that hard:

The real test will come when I incorporate people’s answers back into this. Anyone else tried to do something like this before?

As far as the software I’m actually using. I once tried out MindManager from MindJet. I loved it because it was well integrated with my tablet (i.e. I could use my pen to think, which seems to be how my mind works). It does way more than I ever needed – and I didn’t like the price tag so much, either. So I’ve been using FreeMind – free and java based and not really well integrated with Windows, but it works.

Microsoft Research just posted the Cornell Messenger Series of lectures by Feynman. He is excellent. I’d never seen him speak before this – his wry wit – many of the jokes he uses to teach seem rather too fitting given today’s world. I’ve not finished watching them yet – but they are just too good to pass up. The first lecture seems to be fairly general – he is talking about what a physical law is and what it means – so I suspect anyone interested in science would be able to get something out of these.

The site is called Project Tuva. It will require some time. 🙂 And for the places that the audio isn’t totally clear they have a transcript that runs underneath the video. Spotted on the science feed of the Register.